The invention relates to the directing of an incident optical beam to a track of information on a dynamic medium, and, more particularly, to the control and determination of the positioning error of the incident beam with respect to the track. The invention applies advantageously but not limitingly to digital discs, especially compact discs, e.g. Compact Disc Read Only Memory (CDROM), and most particularly to multifunction digital discs such as the Digital Versatile Disc (DVD) storing image data in a compressed manner, for example.
A digital disc includes a single spiral track whose relief is representative of the binary information stored on the track of the disc. The track of the disc is illuminated by an incident optical beam, for example a laser spot, and several photodetectors, for example four, detect the reflection of the light beam on the disc. The optical pickup formed by the photodetectors then delivers four elementary signals delivered respectively by the four photodetectors, as well as an overall signal, or useful signal, equal to the sum of the four elementary signals, and from which the binary information read from the track is extracted.
The directing of the optical beam to the track of the rotating disc is performed exclusively on the basis of the four elementary signals delivered by the photodetectors. In an analog approach: the signals are summed in pairs so as to form two signals which are equalized in an analog equalizer before being shaped, by comparison with a threshold, in two comparators. The two signals thus shaped are mutually phase-shifted if the laser spot is not situated on the track. The phase difference between these two signals is then detected, which phase difference corresponds to the positioning error of the beam with respect to the track. This positioning error is then used conventionally in a servo-control loop to modify the incident optical system and direct the optical beam back to the track.
Generally, an analog approach of this type has the drawback of requiring a considerable number of analog components which results in a larger, bulkier system. Moreover, this number of components is even larger when the useful band of the signals contains high frequencies, thus leading in particular to high consumption. Moreover, as the technology advances, the modification and production of new components of the device require considerable design and production time.
Another known approach includes using a digital approach to sample the signals emanating from the photodetectors before digital processing which includes, e.g., the calculation of the phase shifts. In order to avoid spectral aliasing during sampling, the sampling frequency must be at least twice as high as the maximum frequency of the frequency band of the useful signal containing the information. When the sampling frequency is twice the maximum frequency of the useful band, the number of samples does not make it possible to obtain, through straightforward interpolation between these samples, a correct value of phase shift in the high frequency ranges close to the frequency maximum. The approach includes digitally reconstructing all the signals from the samples. However, this requires digital processing operations which are complex and expensive to implement, e.g. Viterbi decoding processing.
An object of the invention is avoid the drawbacks of the conventional approaches discussed above. This and other objects are achieved by providing a simple digital solution using straightforward interpolation between samples to calculate the positioning error of the beam with respect to the track of the rotating disc; and, more specifically, when the maximum frequency of the useful band of the information is equal to half the sampling frequency.
Such a configuration occurs in certain applications when the speed of rotation of the dynamic medium is very high, for example 12xc3x97 (a speed of rotation of 1xc3x97 corresponding to 4 m/s). Maximum frequencies of around 60 MHz are then obtained. Moreover, with the present semiconductor technologies, for example, 0.25 micron technology, certain components cannot work correctly at frequencies above 120 MHz. The sampling frequency is therefore limited to this frequency.
The invention therefore provides a process for directing an incident optical beam to a track of information on a dynamic medium. According to a general characteristic of the invention, the beam reflected by the medium (for example, the DVD disc) is picked up by an optical pickup comprising several photodetectors (at least two, and preferably four). The elementary signals respectively delivered by the photodetectors are used to formulate two secondary signals, sampled and filtered by a low-pass filter having a cutoff frequency at most equal to a quarter of the sampling frequency. The mutual phase shift between these two sampled and filtered secondary signals is representative of the positioning error of the beam with respect to the track. Moreover, the determination of a value of the mutual phase shift includes the selecting, for each secondary signal, of at least one pair of samples situated outside a predetermined amplitude range around a predetermined threshold (for example the value zero). These two pairs of samples make it possible to tag, respectively for the two secondary signals, two transitions of these secondary signals with respect to the threshold and corresponding to one and the same direction of crossing of the threshold. Additionally, two transitions are determined by interpolation from the selected samples, and the time gap between the two transitions is also determined.
The use of a low-pass filter with a cutoff frequency which is at most equal to a quarter of the sampling frequency makes it possible to obtain a sufficient number of samples to calculate the phase shift by straightforward interpolation. This being so, the low-pass filter eliminates the high frequencies from the frequency spectrum. This therefore results in a theoretical deleting of these frequencies, and in practice, an appreciable local attenuation of the amplitude of the signals. In practice, the sampled and filtered signals are noisy and, by tagging, for each secondary signal, a transition of this signal with respect to a threshold (for example the value zero), using at least one pair of samples situated outside a predetermined amplitude range around this threshold, it is possible to ignore samples whose levels or amplitudes might be situated inside this predetermined range and which if not ignored might lead to erroneous obtainings of transition due to the presence of the noise.
Of course, the person skilled in the art can choose a cutoff frequency for the low-pass filter of lower than a quarter of the sampling frequency. The adjusting of this cutoff frequency of the low-pass filter will be performed by the person skilled in the art as a function of the application and of the accuracy which are desired. In practice, it is preferable to choose a cutoff frequency for the low-pass filter which is not less than a fifth of the maximum frequency of the useful band, i.e. a tenth of the sampling frequency, so as not to eliminate too large a number of samples, which would then lead to a degradation in the accuracy of the phase-shift calculation.
Likewise, the person skilled in the art will be able to adjust the value of the amplitude of the range around the predetermined threshold as a function of the application and in particular of the noise level of the signals. By way of indication, an experimental way of determining the value of the amplitude of the range includes, when calibrating the system, in examining the changes in the phase shift (Positioning error) in open loop. Indeed, the person skilled in the art is aware that by reason of the eccentricity of the disc, the theoretical curve of the phase shift, i.e. of the positioning error, exhibits a sawtooth configuration in open loop. Too small an amplitude value for the range then leads to a very noisy calibration curve which hardly resembles the theoretical curve. Conversely, too high an amplitude value for the range leads to the obtaining of too small a number of values obtained for the positioning error, likewise not making it possible to recover the theoretical curve for the change in positioning error.
It has been observed that it is preferable for the amplitude of the range to be at least equal to the product of the maximum amplitude of the secondary signals times the noise/signal ratio, and likewise preferably less than half the maximum amplitude of the secondary signals.
The invention is based on the observation that when the majority of the information contained in the signals has frequencies situated between the cutoff frequency of the low-pass filter and the sampling frequency, it nevertheless turns out to be possible to circumvent this information when calculating the phase shift, and hence the positioning error, and to use only the information having frequencies below the cutoff frequency of the low-pass filter, to calculate the positioning error. Thus, given the fact that the directing of the beam to the track is performed at low frequency (for example of the order of a few tens of kHz) with respect to the maximum frequency of the signals emanating from the photodetectors, the accuracy obtained according to the invention in calculating the phase shift, by taking into account only some of the samples, is amply compatible with this low-frequency directing. Thus, for a DVD spinning at 12xc3x97, 70% of the frequencies lie above 30 MHz and are between 30 and 60 MHz. The invention therefore makes it possible to use only 30% of these frequencies for the calculation of the positioning error.
Thus, the invention makes it possible, in a very simple manner, and by using interpolations, for example linear interpolations, for calculating the transitions, to direct with an accuracy which is entirely compatible with the required demands, an optical beam to a track of a rotating medium of which the signals emanating from the photodetectors contain information at frequencies of up to half the sampling frequency. Of course, the invention would also apply to discs spinning at much lower speeds and for which the frequencies of the useful band would not exceed a quarter of the sampling frequency. The number of selectable samples would then simply be greater.
According to one embodiment of the invention, for each secondary signal, the selected samples allowing the tagging of the transition comprise a pair of samples situated on either side of the predetermined threshold, outside the predetermined range, and respectively forming a local minimum and a local maximum which follow the secondary signal. Moreover, to minimize the errors in selecting, the order of the temporal occurrence of the local minimum and of the local maximum must be the same for the two pairs of samples relating to the two secondary signals. In other words, for a secondary signal, if a local minimum is selected first followed by a local maximum so as to tag a transition of this secondary signal, the selecting of the two local extremes of the other secondary signal will then be valid only if, for this other secondary signal, a local minimum is also tagged first followed by a local maximum.
It would of course be possible, if the sampling frequency is sufficiently high, to use only these two local extremes to calculate each transition of the secondary signal. This being so, it is preferable, with an objective of further increasing the accuracy of calculation of the transition of the secondary signal with respect to the threshold, for the determining of the transition, for each secondary signal, to comprise the storing of the pair of samples respectively forming a local minimum and a local maximum which follow the secondary signal. The intermediate samples situated between these extreme samples may also be stored. Next, the two intermediate samples situated on either side of the predetermined threshold and in the-neighborhood of this threshold are selected from among these intermediate samples. An interpolation is then performed, for example a linear interpolation, between these two selected intermediate samples to obtain a calculated sample whose level corresponds to the threshold. This calculated sample, whose temporal occurrence may readily be determined, then manifests for this secondary signal the transition with respect to the threshold.
The variation using the local extremes for the selecting of the pair of samples situated outside the predetermined amplitude range around the predetermined threshold and making it possible to tag the transition of a secondary signal is merely one possibility of selection. Another possibility for selecting, for each secondary signal, the pair of samples situated outside the predetermined amplitude range around the predetermined threshold, to tag a transition for this secondary signal, is to use the overall signal containing the information and delivered by the pickup. This overall signal, which is in practice the sum of the elementary signals delivered by the photodetectors, has up to now been used only for the extraction of the data. However, it has been observed that the transitions of this overall signal with respect to the threshold are always situated between the two secondary signals and in the middle of them. Thus, this overall signal, and more especially these transitions with respect to the predetermined threshold, can serve as a phase reference. More precisely, by selecting two samples which almost symmetrically flank a transition of the overall signal, one ensures, on the one hand, that these two selected samples indeed belong to the two secondary signals, and, on the other hand, that the calculation of the phase shift on the basis of these two selected samples will lead to a correct estimate of the positioning error.
Thus, the use of this overall signal delivered by the pickup and containing the information might make it possible to select the pair of samples situated outside the predetermined range, and also makes it possible to control the selecting of these samples if they have been, for example, selected by the method of local extremes. Indeed, the two samples calculated by interpolation must then likewise almost symmetrically flank a transition of the overall signal, to within a tolerance.
The subject of the invention is also a device for directing an incident optical beam to a track of information on a dynamic medium. According to a general characteristic of the invention, this device comprises a pickup able to pick up the beam reflected by the mobile carrier and comprising several photodetectors. The device further includes a first processing stage linked to the pickup and comprising sampling means and low-pass filtering means having a cutoff frequency at most equal to a quarter of the sampling frequency. This first stage is able to use the elementary signals respectively delivered by the photodetectors to formulate two secondary signals, sampled and filtered, whose mutual phase shift is representative of the positioning error of the beam with respect to the track.
The directing device also comprises a second processing stage, linked to the output of the first stage, and comprising a selector for selecting, for each secondary signal, at least one pair of samples situated outside a predetermined amplitude range around a predetermined threshold. The selected samples make it possible to tag, respectively for the two secondary signals, two transitions of these secondary signals with respect to the threshold and corresponding to one and the same direction of crossing of the threshold. Also, a formulating device determines the two transitions by interpolation from the selected samples, and a calculation device determines the time gap between the two transitions, to determine a value of the mutual phase shift.
According to one embodiment of the invention, the second processing stage comprises a memory for storing the selected samples for each secondary signal. Moreover, the selected samples allowing the tagging of the transition comprise a pair of samples situated on either side of the predetermined threshold, outside the predetermined range, and respectively form a local minimum and a local maximum which follow the secondary signal. The order of the temporal occurrence of the local minimum and of the local maximum being the same for the two pairs of samples relating to the two secondary signals.
The formulating device is able to store the intermediate samples situated between these extreme samples, for each secondary signal, in the memory. Then, the two intermediate samples situated on either side of the predetermined threshold and in the neighborhood of this threshold, are selected from among these intermediate samples. The formulating device may further comprise an interpolator for interpolating between the two selected intermediate samples of each secondary signal to determine, for each secondary signal, a calculated sample whose level corresponds to the threshold. Finally, the calculation device determines the positioning error from the two calculated samples.
It is moreover particularly advantageous, with the objective of further circumventing the errors due to noise, for the second processing stage to comprise a median filter, for example of length three, linked to the output of the calculation device, and followed by an interpolator filter.